1. Field of the Invention
The present invention relates generally to an attitude reference system. More specifically, the present invention relates to an integrated reaction wheel assembly (RWA) and fiber optic gyro (FOG) that may be used in the aerospace industry.
2. Background of the Invention
As is well known, a FOG operates on a principal known as the Sagnac effect, which describes what happens when two beams of light travel in opposite directions around the same closed path. If the path is not rotating, the two beams complete their circuit and arrive back at the starting point at the same instant. When the structure is rotating about an appropriate axis, however, the two beams traverse different path lengths, i.e. the beam traveling in the direction of rotation travels a longer path and vice versa. This difference is small, but it can be detected and measured to indicate rotation rate.
When light moves through a material medium, it remains decoupled from the motions of that medium. Unlike water flowing through a pipe, light in an optical fiber is not dragged along when the fiber moves. Specifically the motion of the light waves is unaffected when a loop of fiber carrying counterpropogating beams begins to rotate around the axis passing through the plane of the loop. If two beams are launched simultaneously from an aperture initially at the twelve o'clock position of the loop, after one revolution, the beams come together again at that launch location.
If, however, the loop is rotating clockwise, the aperture will first encounter the counterclockwise beam somewhat earlier, for example at the one o'clock position, and will not catch up with the clock wise beam until later, for example the two o'clock position. Thus, to get back to the aperture, one beam has to follow a longer path than the other. The difference in path length is proportional to the loop's rate of rotation. Measuring this difference is the basis of a FOG.
A FOG has a long physical path length defined by many turns of optical fiber wound into a coil. The beam is divided and injected into opposite ends of the coil, makes a single pass through it, and comes out the other end. Then the two beams are recombined, forming an interference pattern. Since the beams trace exactly the same physical path, although in opposite directions, the interference should be fully constructive in the absence of a Sagnac effect. Rotation changes the path lengths and the phases of the interfering beams, thus producing changes in the fringe intensity that are proportional to the rotation rate.
Longer light paths in a FOG increase the instrument's sensitivity. For fiber wound into a round coil, the magnitude of the Sagnac effect is proportional to the rotational velocity and to the product of the coil diameter and the length of optical fiber on the coil. A typical FOG designed for aircraft navigation will use about 1,000 meters of optical fiber wound in a coil with a mean diameter of about six centimeters.
The aerospace market has a need for a low inertial noise attitude reference system that offers long life and increased inertial performance (lower noise) while minimizing mass. Current systems utilize a separate inertial reference unit (IRU) assembly, which can add additional mass to a satellite.
In addition to IRUs, satellite manufacturers separately install multiple reaction wheel assemblies (RWAs) on a spacecraft to impart rotational rates about the craft's three axes. Gyro packages or IRUs are employed to measure input rates about these three axes. The spacecraft guidance and control processor typically closes all attitude control loops, using information to and from both the IRU and RWA devices. Having both of these devices is expensive, results in increased mass, and reduces the already limited space available on, for example, satellites. These problems may be exacerbated when improved FOG performance is desired because increased length-diameter product results in increased mass or volume.